1. Field of the Invention
The invention in general relates to electronic probes and other instrumentation amplifier circuits which may be used to pass analog signals over a wide band width of frequencies with little distortion, and more particularly to such an amplifier that is programmable.
2. Statement of the Problem
In the field of instrumentation amplifiers, such as amplifiers for electronic test probes, oscilloscopes, and other instruments, the primary requirement is that amplifiers must handle analog signals with little distortion over a wide band-width of frequencies. Amplifiers that may be used for multiplexing are rare, or non-existent, in the field, since multiplexing is usually thought of as inconsistent with the primary requirement. That is, multiplexing is usually associated with trading off at least some signal fidelity and band width in return for the ability to handle many signals simultaneously or alternatively.
It is well-known that an amplifier may be made by tying together an "emitter pair". Such amplifiers usually are implemented in bipolar technology by connecting the emitters of two bipolar transistors. When the emitter pair is connected to the low or ground voltage through a resistor, often designated as R.sub.E, the collectors of the transistors are connected to the high voltage through resistors, the input is provided to the bases of the transistors, and the output is taken from the collectors, the amplifier is known as a differential amplifier. When the resistor R.sub.E is replaced by a third transistor which provides current-source biasing of the first two transistors, the amplifier is known as an operational amplifier or op-amp. It is also well-known to use feedback in such amplifiers by tying the output collector to the input base of the same transistor. The feedback reduces the gain of the amplifier but increases the band width over which low distortion is possible.
While operational amplifiers with feedback are commonly used as amplifiers, it has generally been thought that they do not lend themselves to multiplexing. Multiplexing requires positive switching; that is a circuit that is on must be positively on and a circuit that is off must be positively off, no matter what stray signals are seen on their inputs and outputs. Since the function of operational amplifiers is primarily determined by the biasing provided by the ground and power supply, i.e. the relative voltages applied to the emitters and collectors, stray signals applied through the ground and power supply terminals can alter the on or off state of such amplifiers. Since multiplexing inherently requires many similar adjacent circuits to be connected to the same ground and power supply, feedback op-amps would not appear to be compatible with multiplexing. Further, the very thing that makes feedback op-amps useful for low distortion over a wide bandwidth, the feedback, also tends to make them susceptible to being influenced by stray signals on their outputs. Since it is in the nature of multiplexing that signals from one multiplexing channels show up at the output of adjacent multiplexing channels, feedback op-amps would not seem compatible with multiplexing for this reason also.
For the above reasons the instrumentation art has up to now essentially been limited to single channel systems. When requirements demanded more than one channel, then a number of independent circuits, i.e. a number of single channel systems, were simply placed side by side. This is expensive, since each channel is a separate sophisticated instrumentation amplifier.
As electronic systems have become more and more complex, with ever higher numbers of elements, such as pins on circuit packages, it has become either very time consuming, very expensive, or both to probe state-of-the-art circuit packages with essentially single channel instrumentation. Thus an instrumentation amplifier that provides multiplexing capability with low distortion over a wide band width is highly desirable.